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Descriptions

The detection of subtle variations in mineral chemistry in zoned hydrothermal
alteration associated with the formation of porphyry copper deposits by short-wave
infrared spectroscopy and rock chemistry are potentially valuable vectoring tools for
mineral exploration. In order to correctly interpret the data collected by these methods,
results must be calibrated by mineral data. Hydrothermal white mica, illite and chlorite
grains were sampled from the Ann-Mason porphyry copper deposit in the Yerington
district, Nevada, a Middle Jurassic porphyry copper system extended and tilted ~ 90° to
the west. Mineral compositions vary spatially and record interactions with chemically
distinct hydrothermal fluids over a vertical distance of ~5 km and a lateral distance of ~2
km from the ore center. Data suggest short wave infrared spectroscopy and bulk rock
geochemical sampling can detect changes in mineral chemistry related to ore deposit
formation but both methods have limitations.
To relate short wave infrared spectroscopy to mineral compositions, spectra from
rock samples were measured. Characteristic features commonly used to identify white
mica, illite and chlorite were compared to chemical compositions of mineral grains from
34 samples were determined by electron microprobe analysis. Results demonstrate short
wave infrared spectroscopy can be used to detect changes in the aluminum content of
micas using the wavelength of the 2200 nm feature and may be used to map fluid pH
gradients in rocks with muscovite or illite-bearing assemblages. The following
compositional characteristics of white mica/illite were observed in the short wave
infrared spectra: (1) an increase in the wavelength of the Al-OH absorption at ca. 2200
nm that is positively correlated with Fe+Mg+Mn (apfu) content and negatively correlated
with total Al (apfu) corresponding to Tschermak substitution in both muscovite and illite,
and (2) a decrease the wavelength of the ca. 2200 nm absorption to values below 2193
nm attributed to an increase in Na content (apfu) and the presence of paragonite
intergrown with muscovite. For this sample set, illite cannot be distinguished from
muscovite using short wave infrared spectroscopy. The proportion of Fe:Mg in the
octahedral site in chlorite could not be identified in short wave infrared spectra of rocks
using the wavelength of the 2350 nm feature and may have been obscured by coexisting
highly reflective clays and micas.
To compare trace metal gradients in rocks and minerals, trace metal
concentrations of more than 600 altered rock samples, collected in a broad geochemical
sampling campaign, were measured using inductively coupled plasma-mass spectrometry
and inductively coupled plasma-atomic emission spectroscopy. Trace element contents
of rocks were compared to trace metal contents of hydrothermal white mica, illite and
chlorite determined by laser ablation-inductively coupled plasma-mass spectrometry from
a set of 34 samples. Results show Cu, Mo, Te, Se, Bi, Sb, As, W, Sn, Li and Tl are
enriched in rocks from the zone of potassic, sericitic and shallow-level advanced argillic
alteration that represents the near-vertical pathway of the ore fluid from the mineralized
zone (3.5 km depth) to the near the paleosurface (< 0.5 km). Of these elements, W, Sn
and Tl enrichment in rock can be attributed, at least partially, to increased concentrations
in muscovite and illite. Lithium enrichment can be attributed to increased concentrations
in chlorite and more mafic, chlorite-rich wall-rock lithology at less than 1 km depth.
Zinc, Mn, Co and Ni are depleted in altered rock above the ore zone and redistributed
upward and laterally by both the magmatic hydrothermal fluid and by circulating
sedimentary brines as verified by gradients in chlorite compositions from propylitic
alteration. Copper is detected in chlorites but at concentrations (average 280 ppm) too
low to contribute significantly to the observed Cu anomaly in rock (>1000 ppm).
Chalcophile elements Mo, As, Te, Se, Bi are rarely detected in white mica/illite or
chlorite in concentrations greater than 1 ppm and, in more than 50% of analyses, levels
are below detection.